Effect of Recycled Fine Aggregates on the Performance of Slag-Fly Ash Blended Geopolymer Masonry Mortar

This paper aims to investigate the performance of slag-fly ash blended geopolymer masonry mortar (GMM) made with recycled fine aggregates (RFA). The effect of replacing natural fine aggregates (NFA) with RFA at 0 and 100% replacement rates was examined through three sets of GMM mixes comprising a binder-to-aggregates ratio of 1:2-1:3, a fly ash-to-slag ratio of 2:1-4:1, and a solution-to-binder ratio of 0.5-0.7. The precursor binder was activated using sodium silicate and sodium hydroxide at a mass ratio of 1.5. GMM mixes were evaluated for fresh and hardened properties. Test results showed that the flow was reduced by up to 12% upon 100% RFA replacement, with higher loss in mixes made with a higher fly ash-to-slag ratio. Meanwhile, RFA mixes had faster initial setting times than the control mix made with NFA, especially with a low binder-to-aggregates ratio of 1:3. For similar RFA replacement, the 28-day compressive strength decreased by up to 73%. The highest strength loss was noted for the mix made with a fly ash-to-slag ratio of 4:1. Yet, despite the deficit in performance due to RFA incorporation in GMM, all mixes complied with international standards for masonry applications. Such research findings provide evidence of the viability of utilizing RFA in cement-free masonry mortar, thereby contributing to enhancing the sustainability of the construction industry by conserving non-renewable natural resources and recycling wastes.